PALM HARBOR, Fla. (AP) — Congressman Connie Mack has been working the Republican National Convention in hopes of injecting energy into his campaign to unseat Democratic Sen. Bill Nelson, but he won't be mentioning the race Thursday in his biggest moment of the week.

Mack will be among the speakers leading up to Mitt Romney's speech to close out the convention, a role that's a reward for campaigning with Romney well ahead of Florida's presidential primary. It provided Romney with a key win on his way to securing the nomination. Mack also backed Romney in his failed 2008 campaign.

Building up to the high-profile speaking role, Mack spent the week doing interviews with local and national media and attending fundraisers and functions ranging from a salsa dance party hosted by the American Conservative Union to a fundraiser he held featuring two of Romney's sons.

It culminated in a short speech before Florida delegates Thursday morning, in which he stressed that if he beats Nelson, it could allow Republicans to gain control of the Senate, a remark that brought cheers.

"I want to make sure that we understand the contrast there is in this race," Mack said.

As he has done often while campaigning, Mack said there are big differences between him and Nelson. Mack portrayed Nelson as someone who votes for President Barack Obama's agenda nearly all the time, including the health care overhaul that Mack and other Republicans vow to repeal. He also said Nelson supports tax hikes while he wants to cut taxes.

The Nelson campaign said Mack's claims are false.

"By every credible measure, Bill's rated as a moderate or centrist. He supports the president when he agrees with him; and, he opposes him when he disagrees with him. He's fought the White House over a number of issues, like, the response to the BP oil spill and having a robust manned space program," Nelson spokesman Paul Kincaid said in an email, adding that Nelson isn't the tax hiker Mack says he is. He said Nelson has supported numerous tax cuts, including voting to extend the Bush-era tax breaks and to repeal the estate tax.

Mack, though, was backed up by former House Speaker Newt Gingrich, who spoke immediately after Mack at the delegation breakfast.

"You must defeat Bill Nelson," Gingrich said. "The central issue in the Senate should be that the Bill Nelson who comes home isn't the Bill Nelson who votes. It is fundamentally dishonest for Bill Nelson to come home and pretend that he's anything but a liberal because when he's in Washington he consistently votes with Obama."

Mack said Nelson has supported tax increases while he wants to cut taxes. And Mack said he wants to repeal President Barack Obama's health care overhaul, while Nelson voted for it.

And he stressed that the balance of power in the Senate is online.

Mack, however, won't be talking about Nelson or his Senate campaign Thursday night when he speaks at the convention. Mack's address will be part of the buildup to Republican presidential candidate Mitt Romney's speech.